LTE Physical Structure
A 3GPP (3rd Generation Project Partnership) LTE (Long Term Evolution) supports a type 1 Radio Frame Structure that is applicable to an FDD (Frequency Division Duplex) and a type 2 Radio Frame Structure that is applicable to a TDD (Time Division Duplex).
FIG. 1 illustrates the structure of a type 1 radio frame. The type 1 radio frame consists of 10 subframes, and each subframe consists of 2 Slots.
FIG. 2 illustrates the structure of a type 2 radio frame. The type 2 radio frame consists of 2 half frames, and each half frame consists of 5 subframes, a DwPTS (Downlink Pilot Time Slot), a Guard Period (GP), and an UpPTS (Uplink Pilot Time Slot). Herein, one subframe consists of 2 slots. The DwPTS is used for initial cell search, synchronization, or channel estimation performed by a user equipment. The UpPTS is used for channel estimation performed by a base station and for uplink transmission synchronization performed by the user equipment. The guard period corresponds to a period for eliminating interference occurring in an uplink due to a multiple path delay of a downlink signal between an uplink and a downlink. More specifically, regardless of the type of the radio frame, one frame is configured of 2 slots.
FIG. 3 illustrates a slot structure of an LTE downlink. As shown in FIG. 3, a signal being transmitted from each slot may be expressed by a Resource Grid, which consists of NRBDLNSCRB number of subcarriers and NsymbDL number of OFDM (Orthogonal Frequency Division Multiplexing) symbols. Herein, NRBDL represents a number of Resource Blocks (RBs) within a downlink, NSCRB represents a number of subcarriers configuring one RB, NsymbDL and represents a number of OFDM symbols included in a downlink slot.
FIG. 4 illustrates a slot structure of an LTE uplink. As shown in FIG. 8,
a signal being transmitted from each slot may be expressed by a Resource Grid, which consists of NRBULNSCRB number of subcarriers and NsymbUL number of OFDM (Orthogonal Frequency Division Multiplexing) symbols. Herein, NRBUL represents a number of Resource Blocks (RBs) within an uplink, NSCRB represents a number of subcarriers configuring one RB, and NsymbUL represents a number of OFDM symbols included in an uplink slot.
A Resource Element is a resource unit that is defined by indexes (a, b) within the downlink slot and the uplink slot. Herein, “a” indicates an index within a frequency axis, and “b” represents an index within a time axis.
FIG. 5 illustrates the structure of a downlink subframe. Referring to FIG. 5, in a subframe, a maximum of 3 OFDM symbols located at the beginning of a first slot correspond to a control region allocated to control channel(s). The remaining OFDM symbols correspond to a data region allocated to Physical Downlink Shared Channel(s) (PDSCH(s)). Examples of a downlink control channel used by a 3GPP LTE may include a PCFICH (Physical Control Format Indicator Channel), a PDCCH (Physical Downlink Control Channel), a PHICH (Physical Hybrid ARQ Indicator Channel), and so on.
Definition of the Multiple-Input Multiple-Output (MIMO) Technology
As an abbreviation for Multiple-Input Multiple-Output, MIMO refers to a method evolved from the conventional method of using only one transmit antenna and only one receive antenna, wherein the method can enhance transmission and reception data efficiency by adopting multiple transmit antennas and multiple receive antennas. More specifically, this corresponds to a technology that can increase the capacity or enhance the performance by using multiple antennas in a transmitter or a receiver of a wireless communication system. Hereinafter, the term MIMO will be referred to as multiple antennas.
A multiple antenna technology refers to an application of the technology of completing a message by gathering (or collecting) a plurality of data segments received from multiple antennas without relying on a single antenna path in order to receive the message. Instead, the MIMO technique may combine a plurality of data segments that is received through a plurality of antennas, thereby receiving the entire data. Since the multiple antennas technology may enhance data transmission rate (or speed) within a specific range or increase the system range with respect to a specific data transmission rate (or speed), the multiple antennas technology corresponds to a next generation mobile communication technology that can be broadly used in mobile communication terminals and relay stations. This technology is being highly recognized as a promising next generation technology that can overcome the problem of limited transmission amount in mobile communication.
FIG. 6 illustrates a block view showing the structure of a general multiple antennas (MIMO) communication system. As shown in FIG. 6, if the number of transmit antennas is increased to NT and the number of receive antennas is increased to NR at the same time, unlike in the case wherein multiple antennas are used only in the transmitter or the receiver, a logical channel transmission capacity increases in proportion with the number of antennas. Therefore, the transmission rate may be enhanced, and the frequency efficiency may be drastically enhanced. The transmission rate respective to the increase in the channel transmission capacity may be increased as much as a value of a maximum transmission rate (Ro) multiplied by a rate increase ratio (Ri) when logically using a single antenna.Ri=min(NT,NR)  Equation 1
For example, a MIMO communications system using 4 transmit antennas and 4 receive antennas may theoretically gain a transmission rate 4 times greater than that of a single antenna system. After the theoretical capacity increase of such multi antennas system has been proven in the mid 90s, diverse technologies for realizing a substantial enhancement in the data transmission rate is still under active research and development. Moreover, some of the technologies are already being reflected and applied in diverse standards in wireless communication, such as the 3rd generation mobile communications, the next generation wireless LAN, and so on.
Referring to the trend in the many researches on multi antennas up to the most recent research, research and development on a wide range of perspectives have been actively carried out, wherein the fields of research include research in the aspect of information theory associated with multi antennas communication capacity calculation, research in wireless (or radio) channel measurement and drawing out models, research in time-spatial signal processing technology for enhancing transmission reliability and enhancing transmission rate, and so on, in diverse channel environments and multiple access environments.
Channel Estimation
In a wireless communication system environment, due to a multiple path time delay, fading may occur. Herein, the process of compensating for any distortion occurring in a signal due to an abrupt change in the environment caused by such fading and of recovering the transmitted signal is referred to as channel estimation. Generally, in order to perform such channel estimation, channel estimation is performed by using a signal that is mutually known by the transmitting end and the receiving end. A signal that is mutually known by the transmitting end and the receiving end is referred to as a pilot signal or a reference signal (hereinafter referred to as RS).
In a wireless communication system using an orthogonal frequency division transmission method, there exist a method of allocating a reference signal to all subcarriers and a method of allocating a reference signal in-between data subcarriers.
In order to gain the channel estimation performance, a symbol configured only of reference signals, such as preamble signals is used. Generally, when using such symbol, since the reference signal density is high, the channel estimation performance may be enhanced as compared to the method of allocating a reference signal in-between data subcarriers. However, in this case, since the transmission amount of data decreases, the method of allocating a reference signal in-between data subcarriers is used in order to increase the data transmission amount. However, when using this method, since the reference signal density decreases, the channel estimation performance may be degraded. Therefore, an adequate positioning is required in order to minimize such degrading.
The receiver performs channel estimation using reference signals in accordance with the following process. Since the receiver is informed of the information on the reference signal, the receiver estimates channel information between the receiver and the transmitter from the received signal. The receiver may then use the estimated channel information value so as to accurately perform demodulation on the data transmitted from the transmitter.
When it is given that the reference signal transmitted from the transmitter is referred to as p, that the channel information, which the reference signal experiences during transmission of the reference signal, is referred to as h, that a thermal noise occurring in the receiver is reference to as n, and that the signal received by the receiver is referred to as y, the received signal y may be expressed as
y=hp+n. At this point, Since the reference signal p is already known by the receiver, the reference signal p may be used so as estimate channel information (h′) as shown in Equation 2 below.h′+y/p=h+n/p=h+n′  Equation 2
At this point, accuracy in the channel estimation value h′ estimated by using the reference signal p is decided based upon the n′ value. Therefore, in order to accurately estimate the h′ value, it is imperative that n′ converges with 0. And, therefore, channel estimation should be performed by using a large number of reference signals. When channel estimation is performed by using a large number of reference signals, the influence of n′ may be minimized.
Method of Allocating User Equipment Specific Reference Signals in a 3GPP LTE Downlink System
Among the above-described radio (or wireless) frame structure supported by the 3GPP LTE, the structure of a radio frame applicable to FDD will now be described in detail. Herein, one frame is transmitted during a time period of 10 msec, and this frame is configured of 10 subframes. One subframe is transmitted during a time period 1 msec.
One subframe is configured of 14 or 12 OFDM (Orthogonal Frequency Division Multiplexing) symbols, and any one of 128, 256, 512, 1024, 1536, and 2048 may be selected and used as the number of subcarriers for an OFDM symbol.
FIG. 7 illustrates a user equipment specific (user specific) downlink reference signal structure with respect to a subframe using a normal Cyclic Prefix (normal CP), wherein 1 TTI (Transmission Time Interval) has 14 OFDM symbols. Referring to FIG. 7, R5 represents a user specific reference signal, and 1 indicates an OFDM symbol position within a respective subframe.
FIG. 8 illustrates a user equipment specific downlink reference signal structure, with respect to a subframe using an extended Cyclic Prefix (extended CP), wherein 1 TTI (Transmission Time Interval) has 12 OFDM symbols.
FIG. 9 to FIG. 11 respective illustrate a downlink reference signal structure common to all user equipments for a system having 1, 2, and 4 transmit antennas, when 1 TTI has 14 OFDM symbols. Referring to FIG. 9 to FIG. 11, R0 represents a pilot symbol for transmit antenna 0, R1 represents a pilot symbol for transmit antenna 1, R2 represents a pilot symbol for transmit antenna 2, and R3 represents a pilot symbol for transmit antenna 3. In order to eliminate interference of all the other transmit antennas to a transmit antenna, the other transmit antennas do not transmit signal on the subcarrier used for transmission of a pilot symbol by the transmit antenna.
FIG. 7 and FIG. 8 correspond to user equipment specific downlink reference signal structures, which can be used together with the user equipment common downlink reference signals of FIG. 9 to FIG. 11. For example, in OFDM symbols #0, #1, and #2 of the first slot to which control information is transmitted, the user equipment common downlink reference signals of FIG. 9 to FIG. 11 are used. And, in the remaining OFDM symbols, the user specific downlink reference signals are used.
Also, by multiplying a pre-defined sequence (e.g., Pseudo-random (PN), m-sequence, etc.) by a downlink reference signal for each cell, interference caused by a signal of the pilot symbol received by the receiver from a neighboring cell may be decreased, thereby enhancing the channel estimation performance. A PN sequence may be applied by the OFDM symbol within a subframe, and PN sequence may be applied differently according to a cell ID, a subframe number, an OFDM symbol position, and user equipment ID.
For example, in case of the structure of a 1 Tx pilot symbol of FIG. 9, it can be known that 2 pilot symbols of a transmit antenna are used in a specific OFDM symbol including pilot symbols. In case of the 3GPP LTE system, there are systems configured of various types of bandwidths. Herein, the range of bandwidth types is between 6 RB (Resource Block) and 110 RB. Therefore, the number of pilot symbols of 1 transmit antenna included in 1 OFDM symbol is equal to 2×NRB, and a sequence that is used by being multiplied by a downlink reference signal for each cell shall have the length of 2×NRB. At this point, NRB indicates the number of RBs respective to the bandwidth, and a binary sequence or a complex sequence may be used as the sequence. r(m) of Equation 3 shown below indicates an example of a complex sequence.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              max                                -          1                                    Equation        ⁢                                  ⁢        3            
In Equation 1 shown above, NRBmax indicates the number of RBs respective to the maximum bandwidth. Therefore, according to the above-mentioned description, the corresponding number may be decided to be equal to 110, and c may be defined as a PN sequence corresponding to a Gold sequence. Equation 3 may be expressed for a user equipment specific downlink reference signal as following Equation 4.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              PDSCH                                -          1                                    Equation        ⁢                                  ⁢        4            
In Equation 4, NRBPDSCH indicates the number of RBs corresponding to downlink data assigned to a specific user equipment. Therefore, the length of the sequence may vary depending upon the data size assigned to the user equipment.
The above-described structure of the user equipment specific downlink reference signal may transmit only 1 data stream, and, since a simple extension is unavailable, multiple streams cannot be transmitted. Therefore, the structure of the user equipment specific downlink reference signal is required to be extended so that multiple data streams can be transmitted.